The present invention relates to inkjet printers, and also to methods of controlling inkjet printers.
Inkjet printers are based on forming drops of liquid ink and selectively depositing the ink drops on a substrate. Known inkjet printers generally fall into two categories: drop-on-demand (DOD) printers, and continuous-jet (CJ) printers. Drop-on-demand (DOD) printers selectively form and deposit the inkjet drops on the substrate as and when demanded by control signals from an external data source; whereas continuous-jet (CJ) printers discharge a continuous stream of ink drops towards the substrate for printing thereon, the ink drops not to be printed being intercepted by a gutter.
Continuous-jet (CJ) printers are divided into two types of systems: binary, and multi-level deflection (MLD) systems. In binary systems, the drops are either charged or uncharged, and accordingly, either reach or do not reach the substrate at a single predetermined position. In MLD systems, the drops can receive a large number of charge levels, and accordingly can generate a large number of print positions.
Continuous jet printheads employing the Multi-Level Deflection (MLD) technique are very sensitive to the stability of the speed of the stream of drops formed by the nozzle. This sensitivity is related to the time of flight and the path of each of the drops while passing through a high electrical deflection field. Each of the drops is charged with a specific charge, corresponding to the specific required location of the drop on the printed substrate. A good first order approximation of the distance that each drop passes inside the deflection field can be presented by the equation:L=⅓FL+[(⅔FL)2+MD(T)2]1/2  (Eq. 1)Where:                L=distance of flight        FL=the deflection field length        MD=the deflection of the drop from the undisturbed flight path, inside the deflection field. For a specific deflection field geometry and strength, and for a specific drop mass and charge, MD is dependent on T, which is:        T=the presence time of the drop in the deflection fieldT itself can be presented as:T=L/[Vj+ΔVj]  (Eq. 2)Where:        Vj=the nominal drop speed        ΔVj=the speed variation of the dropIt is reasonable to approximate from the above formulas that:MD˜α/Vjn+β/ΔVjn  (Eq. 3)Where:        α, β—are constants        n—equals approximately 8/3.        
It is clear that any speed variation of a certain drop will change the undisturbed flight path (MD) value from the required value.
Most of the printheads which use the MLD technique are single nozzle units. For these printheads, stitching between drops delivered by adjacent nozzles is not required. Thus, the need to keep MD as accurate as possible by reducing the speed variation is minor.
On the other hand, when printing with multiple nozzle printheads and when very accurate results are required, it is extremely important to minimize MD errors.
Most of the printers currently available in the market use gear pumps in order to generate a sufficient pressure in the nozzle chamber. This pressure determines the speed of the drop via the equation:P=AVj2+BμVj+C  (Eq. 4)Where:                P—the chamber pressure        μ—the ink viscosity        A, B, C—constants.        
In practical implementations, the pumping pressure cannot be ideally constant. Even a gear pump tends to create some pressure fluctuations, which are translated into MD errors. There are some solutions for these pressure fluctuations, using pressure dampeners, but in most cases significant MD errors still remain.
Pumps are driven by motors, which can be very precisely controlled. Theoretically, this is sufficient to maintain the chamber pressure constant. On the other hand, the pump efficiency relates to the viscosity of the ink (that can vary due to multiple reasons), thus closing a control loop on the pressure using the variation of the speed of the pump motor is complicated.
When the ink viscosity varies, the speed of the drops (for a certain chamber pressure) might vary, as clear from the above equation. This complicates the control loop mentioned above to a non-reasonable complexity, which makes it non-reliable, and a major contributor to artifact MD errors.
Some of the available printers attempt to avoid the pump regulation issue through an air-cushion, i.e., an “air over ink” device. This device is an air-pressurized tank which has two internal level indicators: One level indicates “full” condition, and the other indicates “empty” condition. The tank is filled with compressed air, which is easy to accurately and precisely regulate using commercial devices. In these devices, the ink pump fills the compressed tank, through a primary port, until the indicator indicates “full”. Meanwhile, the ink is delivered to the printhead through a secondary port at a pressure that is close to the preset air pressure compressing the tank. After the ink level reaches the “full” condition, the ink pump stops and the tank is drained very slowly (as this device supports only one or couple of nozzles) till the indicator indicates “empty”. This causes the ink pump to start again to fill the tank. If the pump is slow enough, the tank is big enough, the drain rate (the consumption of the nozzle or nozzles) is small enough, and the precise air regulator can support the air volume change in the tank without fluctuation (jiggle), the ink pressure in the nozzle chamber will be much more stable than the similar pressure supported by a controlled gear pump solution. In this case, the MD errors will be reduced dramatically.
However, in multiple nozzle devices (some hundreds of nozzles per device), this solution would not be practical. Multiple nozzle devices require a by-pass flow of the ink inside the printhead, in order to maintain a thermal stability in all the nozzle pressure chambers, which ensures viscosity stability while jetting drops from all the nozzles. The practical meaning of this is that only a small percent of the delivered ink volume is jetted, while most of the delivered ink is by-passed and recirculated back to the ink reservoir. In these consumption conditions, the above-described solution would require a large, high volume, costly cylinder, and a large amount of circulated ink in the system, together with a highly accurate air regulator. Some applications require using an elevated temperature of the ink (above the room temperature) in order to avoid variations in the ink (that might cause color variations). Typically, the ink itself must be heated gently to avoid its separation. Large volumes of ink in the system would require enormous conditioning time. All these disadvantages make the above-described pressurized cylinder techniques impractical in a multiple nozzle apparatus.